Digital transmission networks, such as those based on Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) standards, are used extensively for transporting broadband communications signals. Network elements, such as multiplexers, digital cross-connect systems, and the like, are used in these transmission networks to support a number of different applications, including some that involve multiple switching or routing functions. One example of an application with multiple switching functions is "path-in-line" protection switching, also referred to as "virtual rings" or "ring-on-ring", which involves line switching over bi-directional line switched rings (BLSR) and path switching over unidirectional path switched rings (UPSR).
To support these types of applications, some network elements include a routing structure, such as a switch fabric, to provide the necessary connections for switching signals through the transmission network. Switch fabrics are typically either centralized or distributed, with the former comprising a single, homogeneous fabric that is used for all switching functions and the latter comprising two or more switch fabrics linked together to perform the switching functions. With regard to multiple switching functions, one of the more significant factors affecting overall performance of the switch fabric is the manner in which the switch fabric is controlled. In practice, a common control arrangement is typically used for a centralized switch fabric, while a segmented control arrangement has traditionally only been used for distributed switch fabrics. Consequently, distributed switch fabrics have been the logical choice for supporting applications involving multiple switching functions because a separate control domain and separate switch fabric can be used to support each separate switching function. However, distributed switch fabrics have many disadvantages, including: added cost for the multiple fabrics, slower connections, less design flexibility, and increased physical space and power requirements.
Although centralized switch fabrics offer several advantages over distributed switch fabrics, centralized switch fabrics are known to have problems with applications that involve multiple switching functions. Many of these problems relate to the inherent complexity of the common control arrangement. For example, the control functions for each of the multiple switching functions must be closely coupled in a common control arrangement so that the multiple switching functions can be properly sequenced and prioritized for the single, homogeneous switch fabric. As a result of the extensive coordination required among the various control functions to carry out the sequencing and prioritizing, these common control arrangements typically fail to meet many of the performance requirements for a given application, especially time-based performance requirements.